man/man7/zpool-features.7

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.Dd June 21, 2022
.Dt ZPOOL-FEATURES 7
.Os
.
.Sh NAME
.Nm zpool-features
.Nd description of ZFS pool features
.
.Sh DESCRIPTION
ZFS pool on-disk format versions are specified via
.Dq features
which replace the old on-disk format numbers
.Pq the last supported on-disk format number is 28 .
To enable a feature on a pool use the
.Nm zpool Cm upgrade ,
or set the
.Sy feature Ns @ Ns Ar feature-name
property to
.Sy enabled .
.Pp
The pool format does not affect file system version compatibility or the ability
to send file systems between pools.
.Pp
Since most features can be enabled independently of each other, the on-disk
format of the pool is specified by the set of all features marked as
.Sy active
on the pool.
If the pool was created by another software version
this set may include unsupported features.
.
.Ss Identifying features
Every feature has a GUID of the form
.Ar com.example : Ns Ar feature-name .
The reversed DNS name ensures that the feature's GUID is unique across all ZFS
implementations.
When unsupported features are encountered on a pool they will
be identified by their GUIDs.
Refer to the documentation for the ZFS
implementation that created the pool for information about those features.
.Pp
Each supported feature also has a short name.
By convention a feature's short name is the portion of its GUID which follows the
.Sq \&:
.Po
i.e.
.Ar com.example : Ns Ar feature-name
would have the short name
.Ar feature-name
.Pc ,
however a feature's short name may differ across ZFS implementations if
following the convention would result in name conflicts.
.
.Ss Feature states
Features can be in one of three states:
.Bl -tag -width "disabled"
.It Sy active
This feature's on-disk format changes are in effect on the pool.
Support for this feature is required to import the pool in read-write mode.
If this feature is not read-only compatible,
support is also required to import the pool in read-only mode
.Pq see Sx Read-only compatibility .
.It Sy enabled
An administrator has marked this feature as enabled on the pool, but the
feature's on-disk format changes have not been made yet.
The pool can still be imported by software that does not support this feature,
but changes may be made to the on-disk format at any time
which will move the feature to the
.Sy active
state.
Some features may support returning to the
.Sy enabled
state after becoming
.Sy active .
See feature-specific documentation for details.
.It Sy disabled
This feature's on-disk format changes have not been made and will not be made
unless an administrator moves the feature to the
.Sy enabled
state.
Features cannot be disabled once they have been enabled.
.El
.Pp
The state of supported features is exposed through pool properties of the form
.Sy feature Ns @ Ns Ar short-name .
.
.Ss Read-only compatibility
Some features may make on-disk format changes that do not interfere with other
software's ability to read from the pool.
These features are referred to as
.Dq read-only compatible .
If all unsupported features on a pool are read-only compatible,
the pool can be imported in read-only mode by setting the
.Sy readonly
property during import
.Po see
.Xr zpool 8
for details on importing pools
.Pc .
.
.Ss Unsupported features
For each unsupported feature enabled on an imported pool, a pool property
named
.Sy unsupported Ns @ Ns Ar feature-name
will indicate why the import was allowed despite the unsupported feature.
Possible values for this property are:
.Bl -tag -width "readonly"
.It Sy inactive
The feature is in the
.Sy enabled
state and therefore the pool's on-disk
format is still compatible with software that does not support this feature.
.It Sy readonly
The feature is read-only compatible and the pool has been imported in
read-only mode.
.El
.
.Ss Feature dependencies
Some features depend on other features being enabled in order to function.
Enabling a feature will automatically enable any features it depends on.
.
.de feature
.It Sy \\$2
.Bl -tag -compact -width "READ-ONLY COMPATIBLE"
.It GUID
.Sy \\$1:\\$2
.if !"\\$4"" \{\
.It DEPENDENCIES
\fB\\$4\fP\c
.if !"\\$5"" , \fB\\$5\fP\c
.if !"\\$6"" , \fB\\$6\fP\c
.if !"\\$7"" , \fB\\$7\fP\c
.if !"\\$8"" , \fB\\$8\fP\c
.if !"\\$9"" , \fB\\$9\fP\c
.\}
.It READ-ONLY COMPATIBLE
\\$3
.El
.Pp
..
.
.ds instant-never \
.No This feature becomes Sy active No as soon as it is enabled \
and will never return to being Sy enabled .
.
.ds remount-upgrade \
.No Each filesystem will be upgraded automatically when remounted, \
or when a new file is created under that filesystem. \
The upgrade can also be triggered on filesystems via \
Nm zfs Cm set Sy version Ns = Ns Sy current Ar fs . \
No The upgrade process runs in the background and may take a while to complete \
for filesystems containing large amounts of files.
.
.de checksum-spiel
When the
.Sy \\$1
feature is set to
.Sy enabled ,
the administrator can turn on the
.Sy \\$1
checksum on any dataset using
.Nm zfs Cm set Sy checksum Ns = Ns Sy \\$1 Ar dset
.Po see Xr zfs 8 Pc .
This feature becomes
.Sy active
once a
.Sy checksum
property has been set to
.Sy \\$1 ,
and will return to being
.Sy enabled
once all filesystems that have ever had their checksum set to
.Sy \\$1
are destroyed.
..
.
.Sh FEATURES
The following features are supported on this system:
.Bl -tag -width Ds
.feature org.zfsonlinux allocation_classes yes
This feature enables support for separate allocation classes.
.Pp
This feature becomes
.Sy active
when a dedicated allocation class vdev
.Pq dedup or special
is created with the
.Nm zpool Cm create No or Nm zpool Cm add No commands .
With device removal, it can be returned to the
.Sy enabled
state if all the dedicated allocation class vdevs are removed.
.
.feature com.delphix async_destroy yes
Destroying a file system requires traversing all of its data in order to
return its used space to the pool.
Without
.Sy async_destroy ,
the file system is not fully removed until all space has been reclaimed.
If the destroy operation is interrupted by a reboot or power outage,
the next attempt to open the pool will need to complete the destroy
operation synchronously.
.Pp
When
.Sy async_destroy
is enabled, the file system's data will be reclaimed by a background process,
allowing the destroy operation to complete
without traversing the entire file system.
The background process is able to resume
interrupted destroys after the pool has been opened, eliminating the need
to finish interrupted destroys as part of the open operation.
The amount of space remaining to be reclaimed by the background process
is available through the
.Sy freeing
property.
.Pp
This feature is only
.Sy active
while
.Sy freeing
is non-zero.
.
.feature com.delphix bookmarks yes extensible_dataset
This feature enables use of the
.Nm zfs Cm bookmark
command.
.Pp
This feature is
.Sy active
while any bookmarks exist in the pool.
All bookmarks in the pool can be listed by running
.Nm zfs Cm list Fl t Sy bookmark Fl r Ar poolname .
.
.feature com.datto bookmark_v2 no bookmark extensible_dataset
This feature enables the creation and management of larger bookmarks which are
needed for other features in ZFS.
.Pp
This feature becomes
.Sy active
when a v2 bookmark is created and will be returned to the
.Sy enabled
state when all v2 bookmarks are destroyed.
.
.feature com.delphix device_removal no
This feature enables the
.Nm zpool Cm remove
command to remove top-level vdevs,
evacuating them to reduce the total size of the pool.
.Pp
This feature becomes
.Sy active
when the
.Nm zpool Cm remove
command is used
on a top-level vdev, and will never return to being
.Sy enabled .
.
.feature org.illumos edonr no extensible_dataset
This feature enables the use of the Edon-R hash algorithm for checksum,
including for nopwrite
.Po if compression is also enabled, an overwrite of
a block whose checksum matches the data being written will be ignored
.Pc .
In an abundance of caution, Edon-R requires verification when used with
dedup:
.Nm zfs Cm set Sy dedup Ns = Ns Sy edonr , Ns Sy verify
.Po see Xr zfs 8 Pc .
.Pp
Edon-R is a very high-performance hash algorithm that was part
of the NIST SHA-3 competition.
It provides extremely high hash performance
.Pq over 350% faster than SHA-256 ,
but was not selected because of its unsuitability
as a general purpose secure hash algorithm.
This implementation utilizes the new salted checksumming functionality
in ZFS, which means that the checksum is pre-seeded with a secret
256-bit random key
.Pq stored on the pool
before being fed the data block to be checksummed.
Thus the produced checksums are unique to a given pool,
preventing hash collision attacks on systems with dedup.
.Pp
.checksum-spiel edonr
.
.feature com.delphix embedded_data no
This feature improves the performance and compression ratio of
highly-compressible blocks.
Blocks whose contents can compress to 112 bytes
or smaller can take advantage of this feature.
.Pp
When this feature is enabled, the contents of highly-compressible blocks are
stored in the block
.Dq pointer
itself
.Po a misnomer in this case, as it contains
the compressed data, rather than a pointer to its location on disk
.Pc .
Thus the space of the block
.Pq one sector, typically 512 B or 4 KiB
is saved, and no additional I/O is needed to read and write the data block.
.
\*[instant-never]
.
.feature com.delphix empty_bpobj yes
This feature increases the performance of creating and using a large
number of snapshots of a single filesystem or volume, and also reduces
the disk space required.
.Pp
When there are many snapshots, each snapshot uses many Block Pointer
Objects
.Pq bpobjs
to track blocks associated with that snapshot.
However, in common use cases, most of these bpobjs are empty.
This feature allows us to create each bpobj on-demand,
thus eliminating the empty bpobjs.
.Pp
This feature is
.Sy active
while there are any filesystems, volumes,
or snapshots which were created after enabling this feature.
.
.feature com.delphix enabled_txg yes
Once this feature is enabled, ZFS records the transaction group number
in which new features are enabled.
This has no user-visible impact, but other features may depend on this feature.
.Pp
This feature becomes
.Sy active
as soon as it is enabled and will never return to being
.Sy enabled .
.
.feature com.datto encryption no bookmark_v2 extensible_dataset
This feature enables the creation and management of natively encrypted datasets.
.Pp
This feature becomes
.Sy active
when an encrypted dataset is created and will be returned to the
.Sy enabled
state when all datasets that use this feature are destroyed.
.
.feature com.delphix extensible_dataset no
This feature allows more flexible use of internal ZFS data structures,
and exists for other features to depend on.
.Pp
This feature will be
.Sy active
when the first dependent feature uses it, and will be returned to the
.Sy enabled
state when all datasets that use this feature are destroyed.
.
.feature com.joyent filesystem_limits yes extensible_dataset
This feature enables filesystem and snapshot limits.
These limits can be used to control how many filesystems and/or snapshots
can be created at the point in the tree on which the limits are set.
.Pp
This feature is
.Sy active
once either of the limit properties has been set on a dataset
and will never return to being
.Sy enabled .
.
.feature com.delphix hole_birth no enabled_txg
This feature has/had bugs, the result of which is that, if you do a
.Nm zfs Cm send Fl i
.Pq or Fl R , No since it uses Fl i
from an affected dataset, the receiving party will not see any checksum
or other errors, but the resulting destination snapshot
will not match the source.
Its use by
.Nm zfs Cm send Fl i
has been disabled by default
.Po
see
.Sy send_holes_without_birth_time
in
.Xr zfs 4
.Pc .
.Pp
This feature improves performance of incremental sends
.Pq Nm zfs Cm send Fl i
and receives for objects with many holes.
The most common case of hole-filled objects is zvols.
.Pp
An incremental send stream from snapshot
.Sy A No to snapshot Sy B
contains information about every block that changed between
.Sy A No and Sy B .
Blocks which did not change between those snapshots can be
identified and omitted from the stream using a piece of metadata called
the
.Dq block birth time ,
but birth times are not recorded for holes
.Pq blocks filled only with zeroes .
Since holes created after
.Sy A No cannot be distinguished from holes created before Sy A ,
information about every hole in the entire filesystem or zvol
is included in the send stream.
.Pp
For workloads where holes are rare this is not a problem.
However, when incrementally replicating filesystems or zvols with many holes
.Pq for example a zvol formatted with another filesystem
a lot of time will be spent sending and receiving unnecessary information
about holes that already exist on the receiving side.
.Pp
Once the
.Sy hole_birth
feature has been enabled the block birth times
of all new holes will be recorded.
Incremental sends between snapshots created after this feature is enabled
will use this new metadata to avoid sending information about holes that
already exist on the receiving side.
.Pp
\*[instant-never]
.
.feature org.open-zfs large_blocks no extensible_dataset
This feature allows the record size on a dataset to be set larger than 128 KiB.
.Pp
This feature becomes
.Sy active
once a dataset contains a file with a block size larger than 128 KiB,
and will return to being
.Sy enabled
once all filesystems that have ever had their recordsize larger than 128 KiB
are destroyed.
.
.feature org.zfsonlinux large_dnode no extensible_dataset
This feature allows the size of dnodes in a dataset to be set larger than 512 B.
.
This feature becomes
.Sy active
once a dataset contains an object with a dnode larger than 512 B,
which occurs as a result of setting the
.Sy dnodesize
dataset property to a value other than
.Sy legacy .
The feature will return to being
.Sy enabled
once all filesystems that have ever contained a dnode larger than 512 B
are destroyed.
Large dnodes allow more data to be stored in the bonus buffer,
thus potentially improving performance by avoiding the use of spill blocks.
.
.feature com.delphix log_spacemap yes com.delphix:spacemap_v2
This feature improves performance for heavily-fragmented pools,
especially when workloads are heavy in random-writes.
It does so by logging all the metaslab changes on a single spacemap every TXG
instead of scattering multiple writes to all the metaslab spacemaps.
.Pp
\*[instant-never]
.
.feature org.illumos lz4_compress no
.Sy lz4
is a high-performance real-time compression algorithm that
features significantly faster compression and decompression as well as a
higher compression ratio than the older
.Sy lzjb
compression.
Typically,
.Sy lz4
compression is approximately 50% faster on compressible data and 200% faster
on incompressible data than
.Sy lzjb .
It is also approximately 80% faster on decompression,
while giving approximately a 10% better compression ratio.
.Pp
When the
.Sy lz4_compress
feature is set to
.Sy enabled ,
the administrator can turn on
.Sy lz4
compression on any dataset on the pool using the
.Xr zfs 8
command.
All newly written metadata will be compressed with the
.Sy lz4
algorithm.
.Pp
\*[instant-never]
.
.feature com.joyent multi_vdev_crash_dump no
This feature allows a dump device to be configured with a pool comprised
of multiple vdevs.
Those vdevs may be arranged in any mirrored or raidz configuration.
.Pp
When the
.Sy multi_vdev_crash_dump
feature is set to
.Sy enabled ,
the administrator can use
.Xr dumpadm 8
to configure a dump device on a pool comprised of multiple vdevs.
.Pp
Under
.Fx
and Linux this feature is unused, but registered for compatibility.
New pools created on these systems will have the feature
.Sy enabled
but will never transition to
.Sy active ,
as this functionality is not required for crash dump support.
Existing pools where this feature is
.Sy active
can be imported.
.
.feature com.delphix obsolete_counts yes device_removal
This feature is an enhancement of
.Sy device_removal ,
which will over time reduce the memory used to track removed devices.
When indirect blocks are freed or remapped,
we note that their part of the indirect mapping is
.Dq obsolete
– no longer needed.
.Pp
This feature becomes
.Sy active
when the
.Nm zpool Cm remove
command is used on a top-level vdev, and will never return to being
.Sy enabled .
.
.feature org.zfsonlinux project_quota yes extensible_dataset
This feature allows administrators to account the spaces and objects usage
information against the project identifier
.Pq ID .
.Pp
The project ID is an object-based attribute.
When upgrading an existing filesystem,
objects without a project ID will be assigned a zero project ID.
When this feature is enabled, newly created objects inherit
their parent directories' project ID if the parent's inherit flag is set
.Pq via Nm chattr Sy [+-]P No or Nm zfs Cm project Fl s Ns | Ns Fl C .
Otherwise, the new object's project ID will be zero.
An object's project ID can be changed at any time by the owner
.Pq or privileged user
via
.Nm chattr Fl p Ar prjid
or
.Nm zfs Cm project Fl p Ar prjid .
.Pp
This feature will become
.Sy active
as soon as it is enabled and will never return to being
.Sy disabled .
\*[remount-upgrade]
.
.feature com.datto resilver_defer yes
This feature allows ZFS to postpone new resilvers if an existing one is already
in progress.
Without this feature, any new resilvers will cause the currently
running one to be immediately restarted from the beginning.
.Pp
This feature becomes
.Sy active
once a resilver has been deferred, and returns to being
.Sy enabled
when the deferred resilver begins.
.
.feature org.illumos sha512 no extensible_dataset
This feature enables the use of the SHA-512/256 truncated hash algorithm
.Pq FIPS 180-4
for checksum and dedup.
The native 64-bit arithmetic of SHA-512 provides an approximate 50%
performance boost over SHA-256 on 64-bit hardware
and is thus a good minimum-change replacement candidate
for systems where hash performance is important,
but these systems cannot for whatever reason utilize the faster
.Sy skein No and Sy edonr
algorithms.
.Pp
.checksum-spiel sha512
.
.feature org.illumos skein no extensible_dataset
This feature enables the use of the Skein hash algorithm for checksum and dedup.
Skein is a high-performance secure hash algorithm that was a
finalist in the NIST SHA-3 competition.
It provides a very high security margin and high performance on 64-bit hardware
.Pq 80% faster than SHA-256 .
This implementation also utilizes the new salted checksumming
functionality in ZFS, which means that the checksum is pre-seeded with a
secret 256-bit random key
.Pq stored on the pool
before being fed the data block to be checksummed.
Thus the produced checksums are unique to a given pool,
preventing hash collision attacks on systems with dedup.
.Pp
.checksum-spiel skein
.
.feature com.delphix spacemap_histogram yes
This features allows ZFS to maintain more information about how free space
is organized within the pool.
If this feature is
.Sy enabled ,
it will be activated when a new space map object is created, or
an existing space map is upgraded to the new format,
and never returns back to being
.Sy enabled .
.
.feature com.delphix spacemap_v2 yes
This feature enables the use of the new space map encoding which
consists of two words
.Pq instead of one
whenever it is advantageous.
The new encoding allows space maps to represent large regions of
space more efficiently on-disk while also increasing their maximum
addressable offset.
.Pp
This feature becomes
.Sy active
once it is
.Sy enabled ,
and never returns back to being
.Sy enabled .
.
.feature org.zfsonlinux userobj_accounting yes extensible_dataset
This feature allows administrators to account the object usage information
by user and group.
.Pp
\*[instant-never]
\*[remount-upgrade]
.
.feature org.openzfs zilsaxattr yes extensible_dataset
This feature enables
.Sy xattr Ns = Ns Sy sa
extended attribute logging in the ZIL.
If enabled, extended attribute changes
.Pq both Sy xattrdir Ns = Ns Sy dir No and Sy xattr Ns = Ns Sy sa
are guaranteed to be durable if either the dataset had
.Sy sync Ns = Ns Sy always
set at the time the changes were made, or
.Xr sync 2
is called on the dataset after the changes were made.
.Pp
This feature becomes
.Sy active
when a ZIL is created for at least one dataset and will be returned to the
.Sy enabled
state when it is destroyed for all datasets that use this feature.
.
.feature com.delphix zpool_checkpoint yes
This feature enables the
.Nm zpool Cm checkpoint
command that can checkpoint the state of the pool
at the time it was issued and later rewind back to it or discard it.
.Pp
This feature becomes
.Sy active
when the
.Nm zpool Cm checkpoint
command is used to checkpoint the pool.
The feature will only return back to being
.Sy enabled
when the pool is rewound or the checkpoint has been discarded.
.El
.
.Sh SEE ALSO
.Xr zfs 8 ,
.Xr zpool 8